Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (communication units). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
A cell's radio coverage is provided by a base transceiver station (BTS). Each BTS may contain one or more transceivers (TRX) which can simultaneously receive on one frequency and transmit on another. Communication between a BTS and a mobile communication unit (or mobile station) (MS) typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BTS.
The pair of frequencies assigned for use at the remote site are typically referred to as a radio channel. Downlink transmissions (from BTS to MS) on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions (from MS to BTS) on the radio channel occurs on the second frequency of the pair of frequencies.
The GSM system is a TDM/TDMA system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel assigned to a BTS, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic users (speech or data) in addition to a multiplexed common control channel within the eight TDM slots. Additional, secondary radio channels assigned to the same cell can provide a full complement of eight full rate traffic users (in the 8 TDM slots) per radio channel, since the control channel within the primary radio channel can control allocation of communication resources on secondary radio channels.
Transmissions (control or speech and/or data traffic) from a BTS to an MS, on the downlink, occupy a first TDM slot (downlink slot) on a first frequency of a radio channel and transmissions from a communication unit to a BTS, on the uplink, occupy a second TDM slot (uplink slot) on the second frequency of the radio channel. The uplink slot on the second frequency is displaced in time three TDM slot positions following the downlink slot on the first frequency. The uplink slot on the second frequency is offset 45 MHz lower in frequency than the downlink. The uplink slot and downlink slot (together providing a two-way signal path for a single user) may be referred to as a "communication resource", allocated by the BTS to an MS for exchanging signals. The term "communication resource" also typically includes an associated signalling channel, as for example the GSM specified slow associated control channel used with traffic channels.
Exchanges of paging and setup control information within GSM between MSs and BTSs typically occurs on the common control channel (CCCH) which occupies at least one slot of a primary channel of the BTS. Transmitted by the BTS on the CCCH are distinctive identification signals as well as synchronization and timing information common to all other frequencies and slots of the BTS. CCCH information allows an MS to differentiate between primary and non-primary channels.
Upon activation, an MS scans a set of frequencies in search of CCCH identification signals transmitted from proximate BTSs. Upon detecting a CCCH identification signal the communication unit measures a signal quality factor (such as signal strength) of the identification signal as a means of determining relative proximity of the BTS. Upon completing the scan of frequencies within the set, the MS generally selects the BTS providing the largest relative signal quality factor as a serving BTS. Upon identifying, and locking onto a suitably strong signal (and registering if necessary) the communication unit monitors the selected CCCH for incoming calls. Should the communication unit desire to initiate a call, an access request may be transmitted using the CCCH of the serving BTS.
During normal operation (including during active calls), the MS monitors for, identifies, and measures primary channels of nearby BTSs. If involved in an active call, the MS relays measurement information back to the base site on an associated signaling channel. Through such a process, it is possible for the MS to maintain an association with the most appropriate (proximate) BTS. The process may entail an autonomous switching by the MS to a different BTS, causing perhaps a re-registration by the MS with the system indicating that such a switch has occurred. Alternatively, during an active communication exchange, the MS may be commanded by the system to handover to a more appropriate BTS.
The GSM system specification allows for frequency hopping of the radio channel. Under frequency hopping in GSM, the exchange of communicated signals between the BSS and MS occurs on an indexed frequency which indexes after each TDM frame. As is known, the combination of frequency hopping, in conjunction with error correction coding techniques, results in a significant improvement in reception quality and improved interference robustness due to reduced radio channel fading and noise.
It is necessary, and it is so specified for GSM (see GSM Recommendation 5.02, Section 6.2.4), that the downlink frequency of the primary radio channel for each BTS be non-hopping. A non-hopping primary channel is necessary so that the CCCH of the downlink primary radio channel can act as the beacon for cell selection and measurement by the MS. The need for a beacon represents a significant disadvantage to cells with limited channel availability since it not only imposes a requirement that the CCCH not hop, but likewise imposes a similar requirement on the other seven slots of the primary radio channel.
Non-hopping CCCHs are a serious detriment to systems that are intended for use with low power portable radios. In order to accommodate low power subscriber units, cell spacings of such systems tend to be small. A minimal number of radio carriers are allowed to accommodate the typical capacity requirements of such small areas because of the small service coverage areas and relatively few subscribers. Thus, a relatively high proportion of carriers in the system are used as primary radio channels, and therefore are non-hopping. Low power subscriber units as a consequence do not benefit from the improvements provided by frequency hopping.
A need exists for a method of allowing the use of frequency hopping within cells having a single primary channel while still providing an MS beacon. Such a method would provide at least some of the signal quality benefits of frequency hopping to primary radio channels without diminishing the ability of the MS to easily detect, measure signal strength, and identify nearby BTSs. Such a method would offer the immunity to noise and fading of frequency hopping while preserving the desirable system control attributes inherent with GSM.